A problem exists in obtaining a high-gain semiconductor-based photodetector that operates at specific wavelengths of interest. And in particular, one embodiment of the invention is a high-gain semiconductor heterojunction phototransistor (HPT), of a type based on Group III-V material, responsive to wavelengths in the range of approximately 930 nanometers (nm) to approximately one micrometer; this embodiment has not been heretofore practicably achieved because of a lack of sufficient absorption of the incident radiation.
One method to increase photosensitivity at wavelengths greater than 900 nm is to incorporate InGaAs material into the device. However, because of crystal lattice mismatch between GaAs and InGaAs, the thickness and composition of the InGaAs layers are constrained. This limits the thickness of the InGaAs layer to a thickness that is significantly less than an optical absorption length. As a result, the inclusion of a relatively thin InGaAs layer results in a device with a relatively low gain for wavelengths of interest, that is, in the range of approximately 900 nm to approximately one micrometer.
Unlu et al. report in an article entitled "Resonant cavity enhanced AlGaAs/GaAs heterojunction phototransistors with an intermediate InGaAs Layer in the collector," Appl. Phys. Lett. 57 (8), Aug. 20, 1990, pg. 750-752, a device containing a photosensitive layer that is nearly lattice matched to a GaAs substrate of the device. As a result, the photosensitive layer is substantially unstrained. The device operates at approximately 900 nm, a wavelength in the absorption band of highly doped GaAs substrates, by incorporating a bulk layer of In.sub.0.05 Ga.sub.0.95 As grown on top of a distributed Bragg reflector (DBR), thus providing a strongly asymmetric microresonator. However, the photosensitivity of the GaAs-based HPT cannot be extended substantially beyond 900 nm, because of the low In mole fraction in the InGaAs photo absorbing layers. Also, in this device there is no apparent mechanism to incorporate a highly strained layer.
In an article entitled "Surface-Emitting, Multiple Quantum Well GaAs/AlGaAs Laser with Wavelength-Resonant Periodic Gain Medium" by M. Y. A. Raja et al., Appl. Phys. Lett. Vol. 53, No. 18, Oct. 31, 1988, pg. 1679-1680, there is described Resonant Periodic Gain (RPG). RPG is employed to optimize the extraction of radiation from a vertical-cavity surface-emitting laser structure. To achieve RPG, GaAs quantum wells (QWs) are placed at the anti-nodes of a standing wave field set up by a microresonator.
It is thus an object of the invention to enhance the sensitivity of a photodetector which previously has low or minimal sensitivity to certain wavelengths of interest. This object is achieved by incorporating into the photodetector a resonant cavity having resonant cavity enhancement and Resonant Periodic Absorption between two reflectors.
Another object of the invention is to employ Resonant Periodic Absorption to increase photosensitivity of the photodetector. This object is realized by placing radiation absorbing regions of the photodetector at the antinodes of the standing waves of the light within the resonant cavity. An advantage of this feature is that the photodetector is sensitive at wavelengths greater than the absorption edge of the substrate of the device.
It is yet another object of this invention to provide a photodetector having two reflectors, which may be Distributed Bragg reflectors (DBR), i.e., mirror stacks which form a microresonator for enhancing radiation absorption efficiency, wherein the reflectors are on either end of a resonant cavity. An advantage of this structure is that the photodetector is either sensitive to a specific wavelength range, or attenuates other wavelengths outside of the range of interest.
Another object of the invention is to provide a detector for laser emissions having wavelengths greater than 900 nm, capable of passing through the substrate of the photodetector, such as GaAs, without significant attenuation. This object is achieved by incorporating a highly strained-layer within a photodetector at the antinodes of the standing lights waves set up in the resonant cavity of the photodetector. An advantage of this feature is that the performance of the photodetector is maintained at acceptable levels.